I managed to do this by accident three times today while testing using
the dashboard display, so it's time to action on it.
Touched on in
https://github.com/ppy/osu/discussions/30740#discussioncomment-11345996.
Was also mentioned recently in discord or another discussion explicitly
but I can't find that.
The intention here is to make things more testable going forward.
Specifically, to remove the "back-door" entrance into `BeatmapCarousel`
where `BeatmapSets` can be set by tests and bypas/block realm retrieval.
I'm not sure why this was "fine" for as long as it apparently was,
but what `MusicController` was doing was completely incorrect and
playing with fire (accessing raw managed realm objects), which went
wrong somewhere around - admittedly -
https://github.com/ppy/osu/pull/29917, likely because that one started
*storing* these raw managed realm objects to fields, and you know what
will happen to those after you do a migration and recycle realms.
To attempt to circumvent this, (ab)use `DetachedBeatmapStore` instead.
Which does necessitate moving it to `OsuGameBase`, but it's the simplest
way out I can see. I guess the alternative would be to faff around with
`Live<T>` but it's ugly and I'm attempting to fix this relatively quick
right now.